Motion and Evaporation of Shear-Driven Liquid Films in Turbulent Gases

Detailed measurements of wavy liquid films driven by the shear stress of turbulent air flow are obtained for different air temperatures, air velocities, and flow rates of the liquid. The experimental conditions are chosen from characteristic data of liquid film flow in prefilming airblast atomizers and film vaporization employing combustors. For the measurement of the local film thickness and film velocity a new optical instrument—based on the light absorption of the liquid—has been developed, which can be used at high temperatures with evaporation. The measured data of the gas phase and the liquid film are compared with the results of a numerical code using a laminar as well as a turbulent model for the film flow and a standard numerical finite volume code for the gas phase. The results utilizing the two models for the liquid film show that the film exhibits laminar rather than turbulent characteristics under a wide range of flow conditions. This is of considerable interest when heat is transferred across the film by heating or cooling of the wall. With this information the optical instrument can also be used to determine the local shear stress of the gas phase at the phase interface. Using time-averaged values for the thickness, the velocity, and the roughness of the film, the code leads to relatively accurate predictions of the interaction of the liquid film with the gas phase.